1) Field of the Invention
The present invention relates to an acousto-optic device suitable for use in an optical communication system.
2) Description of the Related Art
An acousto-optical device is a device utilizing an acousto-optic effect that light is diffracted according to a refractive index profile in a substance induced upon receipt of an ultrasonic wave and, for example, includes an AOTF (Acousto-Optic Tunable Filter). In addition to the application to an OADM (Optical Add-Drop Multiplexer) in a WDM (Wavelength Division Multiplexing, wavelength multiplexed) optical communication system, this AOTF is expected to be available for an optical cross connect, an optical switch, an optical modulation and others.
The AOTF is a wavelength filter using the TE–TM mode conversion stemming from the acousto-optic effect which is the interaction between light in a waveguide made in a piezoelectric material such as an LiNbO3 (lithium niobate) substrate (sometimes, which will hereinafter be referred to simply as an “LN substrate”) and an SAW (Surface Acoustic Wave) applied thereonto.
FIG. 25 is a top view illustratively showing an example of a configuration of a common AOTF. This AOTF, generally designated at reference numeral 100, is equipped with polarization beam splitters 102 and 104 formed at former and latter stages in an interactional area on an LN substrate 101 and a mode converter 103 serving as the aforesaid interactional area and comprised of two optical waveguides 103a-1, 103a-2 and a SAW guide 103b acting as a SAW waveguide and formed thereon as a thin film. In addition, formed is a comb-like electrode (IDT: Inter Digit Transducer) 105 capable of, when an ultrasonic wave signal is applied thereto, producing a SAW which is propagated through the SAW guide 103b. 
In the AOTF with this configuration, the input-side polarization beam splitter 102 splits the inputted light into TE light and TM light, and the optical waveguides 103a- and 103a-2 constituting the mode converter 103 separately carries out the TE/TM mode conversion on only a light with a specific wavelength through the use of the SAW from the comb-like electrode 105, with they being multiplexed in the output side polarization beam splitter 104. This realizes the polarization-non-dependency of the device characteristic.
Although the mode conversion in the AOTF 100 shown in FIG. 25 is made in the separate waveguides 103a-1 and 103a-2 according to the polarization component for the purpose of realizing the polarization non-dependency, for the mode conversion, there is a need to use the SAWs having the same characteristic with respect to these waveguides 103a-1 and 103a-2. In the aforesaid AOTF 100 shown in FIG. 25, as the SAW to be applied to the optical waveguides 103a-1 and 103a-2, a SAW stemming from an ultrasonic wave to be applied to the transducer 105 is used in common.
As a well-known technique associated with the present invention, there is a technique disclosed in the patent document U.S. Pat. No. 5,218,653. This patent document discloses a technique of dividing an acoustic waveguide by a gap so that the two acoustic waveguides operate as an oriented coupler.
However, in the aforesaid AOTF 100 shown in FIG. 25, the SAW intensity distribution in a direction perpendicular to the SAW propagation direction shows a waveform having a single maximum point in an area interposed between the optical waveguides 103a-1 and 103a-2 as shown in FIG. 26. Therefore, of the intensity of the SAW to be produced by an ultrasonic wave signal applied to the transducer 105, a relatively low portion is used in the vicinity of the optical waveguides 103a-1 and 103a-2, and the interaction is remote from a high efficiency, which leads to an increase in driving power for the generation of the SAW.